The present inventions relates, to a method and apparatus of interconnecting with a system board for the purposes of testing and/or implementation of engineering changes. More specifically, the present invention relates to an interconnection scheme where access is provided to an area of interest on the system board for probing and/or connecting to signals on a system board or a component thereon.
In the testing of large systems during the initial bring up and including debugging of system hardware, special modifications are typically made to the product. A metal stiffener used to support the large system boards is machined so that an open access is provided to e.g., pins of a Multi Chip Module (MCM) as well as providing access to other points of interest. There are presently two methods used to measure system operations; destructive and nondestructive measurement techniques. These are accomplished either by direct soldering of probe connectors to the system board or by the use of an insulated template and probe arrangement. The first method, direct soldering, provides good high frequency measurements but has many limitations and disadvantages. These limitations and disadvantages include, for example, the requirement that the board must be removed from the test fixture each time a connection is to be soldered on, the number of connections present at any time is limited and the connections are susceptible to mechanical failure (e.g., such as being broken off). The second method, utilizing the probe template, offers a full range of interconnections, by means of holes drilled through a template made of an insulating material, at all signal locations as well as selected ground or voltage reference locations of the MCM. This arrangement is limited to measurements in the 500 MHZ bandwidth region. Thus, while this template arrangement is adequate for error injection and some mid-frequency a.c. measurements, it is not suitable for analysis of high frequency switching noise and circuit operation verification.
Another common problem related to system boards lies in implementing system upgrades and functional enhancements of the system board. Presently such system upgrades and functional enhancements require the system board to be replaced. This leads to expensive component rework, handling, and significant impact of computer availability at both the development lab and customer""s office.
Still yet another problem related to system boards is that in the initial bring up of a machine, it is sometimes necessary to temporary change or repair a nets"" termination. Present methods include a destructive mechanical solution of soldering terminating resistors, tie down to ground or a tie up to a voltage on the system board. Again, any time that a component needs to be attached to the system board, the system board must be removed from the test fixture. This impacts test time, availability of the machine, and the over all schedule of a products"" development.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the method and apparatus of interconnecting with a system board of the present invention. In accordance with the present invention, a system board having a metal stiffener (or other such structure) mounted thereon is provided with an opening in the stiffener to provide access to an area of interest on the system board. A probe test assembly is positioned at the opening and secured to the stiffener when testing is desired to provide access to the pins of the device under test (e.g., a Multi Chip Module (MCM) on the system board). A cover is positioned at the opening and secured to the stiffener at all other times.
The probe test assembly in one embodiment of the present invention (high frequency testing applications) comprises an insulated pattern guide plate and a metal (conductive) probe plate which are positioned at the opening and secured to the stiffener by an insulated frame. The insulated frame insulates the metal probe plate from the stiffener. The plates have a pattern or array of holes corresponding to the pattern of pins on the MCM (i.e., the device under test). The insulated pattern plate protects ground pins in the probe plate from being exposed. In high frequency applications the metal probe plate is part of the measurement system. The metal probe plate has resilient ground terminals pressed into selected holes therein which provide a low impedance ground return path for test measurements. For low bandwidth or d.c. testing applications the pattern plate is eliminated and the probe plate is comprised of an insulation material, whereby the probe plate does not form part of the aforementioned ground return path. Since the probe plate in this alternate embodiment is non-conductive a ground pin is not provided.
Alternatively, a system enhancement device, such as a MCM or Single Chip Module (SCM) having additional Central Processing Units (CPU""s) or other features, may be installed on the system board to enhance the function of the system board, providing the system board has reserved I/O interfaces at the location of the opening in the stiffener. The enhancement device is retained by a frame which is mounted to the stiffener after the cover has been removed.
In accordance with another alternate embodiment of the present invention an interface assembly is positioned at the opening in the stiffener, after the cover has been removed, and is retained and located thereat by the frame. The interface assembly provides for system board engineering change capabilities and functional upgrade capabilities, providing that the system board has reserved MCM pin locations and spare nets which are prewired in the system board. The interface assembly comprises an interface board and an interconnect printed circuit board. A pattern or array of holes corresponding to the pattern of I/O interfaces (pads) on the system board are provided through the interface board. Resilient coaxial probe connectors (pins) are located in selected holes for connecting to signal pads. Double ended ground pins are located in selected holes for providing a return or ground connection. A connector is connected to signal and ground traces/pads on the interconnect circuit board and is receptive to a mating connector to provide access to the this signal and ground pair for testing (or other purposes).